The present invention relates to an X-ray generator that generates the X-ray and extreme ultraviolet (“EUV”) light, and an exposure apparatus having the same.
In manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology, a reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern. It is also important for the fine processing to use the exposure light having a shorter wavelength, to make uniform the light intensity that Koehler-illuminates the reticle, and to make uniform the effective light source distribution as an angular distribution of the exposure light that illuminates the reticle and the wafer.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure. Thus, a projection optical apparatus using the EUV light with a wavelength of about 10 nm to about 15 nm much shorter than that of the UV light (referred to as “EUV exposure apparatus” hereinafter) has been developed. The EUV exposure apparatus typically uses a laser plasma light source. It irradiates a laser beam to a target material to generate plasma for use as the EUV light. The EUV exposure apparatus also typically uses a discharge plasma light source that generates the plasma and generates the EUV light by introducing gas to the electrode for discharging. For example, prior art include Japanese Patent Publications, Application Nos. 2002-174700 and 2004-226244.
However, the laser plasma light source generates not only the EUV light but also flying particles called debris from the target material. In addition, the debris is emitted from the supply mechanism that supplies the target material. The debris also spreads from the electrode material in the discharge plasma light source. The debris causes contaminations, damages, and lowered reflectivity of optical elements, making uneven the light intensity and deteriorating the throughput. Accordingly, U.S. Pat. No. 6,359,969 arranges a debris mitigation system between a light emitting point and a mirror so as to remove the debris.
The debris mitigation system is designed to remove the debris and transmit the EUV light, but actually it shields part of the EUV light and lowers the light intensity and throughput. In addition, the debris mitigation system shields the EUV light of a certain angle range, makes uneven the angular distribution and lowers the imaging performance. For example, FIG. 3 schematically shows a relationship between the light intensity per unit solid angle and the angle from the optical axis near the light source outlet. E1 is energy taken in by the optical system. The minimum angle θ1 is determined, as shown in FIG. 4, by an area shielded by the debris mitigation system, and the maximum angle θ2 is determined by the downstream optical system. Without the debris mitigation system, the minimum angle θ1 is smaller, and the angular uniformity and the light intensity that depends upon a product between the angle and the light intensity improves, but the mirror would get damaged by the debris.